High pressure tubular reactors have become established over the past 80 years as a means of producing low and medium density polyethylene polymers. Such tubular reactors are large scale installations and typically operate at a pressure in excess of 2000 bar and sometimes as high as 3100 bar.
Typically, a high pressure process employing a tubular reactor includes compressing ethylene in a primary compressor and then mixing that ethylene with recycled ethylene and further compressing the ethylene, optionally mixed with modifiers or chain transfer agents and/or comonomers, such as vinyl acetate, in a secondary compressor, heating at least a portion of the compressed ethylene and introducing that heated ethylene into the front end of a tubular reactor, introducing an initiator into the tubular reactor in at least three separate locations, thereby defining at least three reaction zones, allowing the ethylene to polymerize, and cooling the reaction mixture in at least the first two reaction zones, maintaining a pressure drop over the length of the tubular reactor, releasing the reaction mixture through a high-pressure, let-down valve, cooling the reaction mixture and separating the reaction mixture in a product separator into polymer and unreacted ethylene, and recycling unreacted ethylene.
The product separation may be carried out in a single stage. However, two stage separation is generally preferred. In the first stage, known as the high pressure separator, the first separation of polymer from unreacted ethylene is carried out. The separated gas is fed to the high pressure recycle system for return to the secondary compressor. The molten polymer in the bottom of the high pressure separator is decompressed into a second stage, known as a low pressure separator, and almost all of the remaining ethylene, and comonomer and modifier if present, is separated from the polymer and is usually sent to the purge gas or low pressure separator off gas compression system. See, WO 2007/018871 and WO 2012/082674; U.S. Patent Application Publication No. 2015/0011717; U.S. Pat. Nos. 6,596,241; 7,411,026; and 8,466,240; and EP 2 589 612 A, EP 2 746 304 A, EP 2 746 305 A, and EP 2 862 872 A.
For example, FIG. 1 is a schematic of a polymerization plant 1 including an ethylene feed line 2 which supplies fresh ethylene to a primary compressor 3. The ethylene discharged from the primary compressor 3 flows via conduit 4 having a valve 4a to the secondary compressor 5. Also entering the secondary compressor 5 is a stream of fresh modifier(s) and/or optional comonomer(s) and a stream of recycled ethylene. The fresh modifier stream is supplied by a separate modifier pump 6. The recycled ethylene comes from the high pressure recycle system 7.
The secondary compressor 5 is described in more detail below. The secondary compressor 5 discharges compressed ethylene in, for example, five streams 8a, 8b, 8c, 8d, and 8e. Stream 8a may account for 20% of the total ethylene flow. Stream 8a is heated by a steam jacket (not shown) which heats the ethylene, prior to entry into the front end of the tubular reactor 9. The four remaining ethylene side streams 8b, 8c, 8d, and 8e may each enter the reactor as sidestreams. Sidestreams 8b, 8c, 8d, and 8e are cooled. The tubular reactor 9 is also shown with six initiator inlets 10a to 10f which are spaced at intervals along reactor 9 and are fed from an initiator mixing and pumping station 11. The first initiator injection point 10a is just downstream of the front end of the reactor 9 and defines the start of the first reaction zone. Initiator entering through that first initiator inlet 10a combines with the hot ethylene from stream 8a and polymerization begins, raising the temperature of the ethylene as it travels down tubular reactor 9. A heating/cooling jacket (not shown) fitted on reactor 9 cools the reaction mixture and the temperature of the reaction mixture peaks at between, for example, 210 and 350° C., as initiator is consumed and the rate of polymerization begins to fall, and then begins to decline. Entry of the first sidestream 8b cools the reaction mixture further. The second initiator injection inlet 10b is just downstream of the entry point of sidestream 8b and defines the start of the second reaction zone. Once again, the temperature of the reaction mixture rises, peaks and falls as it flows along the tubular reactor 9 with the entry of the second sidestream 8c providing a further rapid cooling prior to entry of initiator at the third initiator inlet 10c, which defines the start of the third reaction zone. The third, fourth, fifth and sixth reaction zones are similar to the second reaction zone except that the sidestreams are optional with regard to the fifth and sixth reaction zones, and therefore the distance between the fifth and sixth initiator inlets 10e and 10f may be relatively long, in order to allow for a greater length of heating/cooling jacket.
Downstream of the sixth initiator inlet 10f and the sixth reaction zone, the tubular reactor terminates in a high-pressure, let-down valve 12.
In the region upstream of the injection point of the first sidestream 8b, the tubular reactor 9 has an initial internal diameter, which may increase downstream of sidestream 8b, and increases further downstream of each subsequent sidestream until a maximum internal diameter of, for example, at least 65 mm, and preferably at least 70 mm is reached in the region downstream of the final sidestream 8e. That internal diameter profile allows the flow rate throughout the tubular reactor 9 to be maintained at about 15 m/s during normal operation under a secondary compressor throughput of 160 tonnes/hour and at an acceptable pressure drop across the reactor.
The high-pressure, let-down valve 12 controls the pressure in the tubular reactor 9. Immediately downstream of the high-pressure, let-down valve 12 is product cooler 13. Upon entry to the product cooler 13, the reaction mixture is in a phase separated state. It exits into high pressure separator 14. The overhead gas from the high pressure separator 14 flows into the high pressure recycle system 7 where the unreacted ethylene is cooled and returned to the secondary compressor 5. The polymer product flows from the bottom of the high pressure separator 14 into the low pressure separator 15, separating almost all of the remaining ethylene, optionally, comonomers and/or modifiers, if present, sometimes collectively called the “off gas” from the polymer. That remaining ethylene and optional components are typically transferred either to a flare or purge system (not shown). The molten polymer flows from the bottom of the low pressure separator 15 to an extruder (not shown) for extrusion, cooling and pelletizing via line 15a. 
Purging or flaring the off gas from the low pressure separator results in a loss of valuable monomers and modifiers, especially, considering the number of and large scale of high pressure polyethylene installations around the world. Accordingly, there remains a need to capture value by recycling components, mainly, monomers and modifiers, of the low pressure separator off gas, as opposed to merely purging or flaring them from the system.